1. Field of the Invention
The present invention relates to a liquid crystal module, and more particularly to a backlight unit for uniformly illuminating a liquid crystal display, and a liquid crystal module using the same.
2. Description of the Related Art
A liquid crystal display device is not a self-luminous display device, and requires a separate light source like a backlight unit to display an image. A liquid crystal display device includes a liquid crystal display panel with liquid crystal cells arranged in a matrix; a liquid crystal module including a backlight unit for irradiating light to the liquid crystal display panel; a drive circuit unit for driving the liquid crystal module; and a case.
The types of devices for the light source of the backlight unit include a cold cathode fluorescent lamp (CCFL); a hot cathode fluorescent lamp (HCFL); an external electrode fluorescent lamp (EEFL); and a light emitting diode (LED). The backlight unit may be classified as either an edge type or a direct type in accordance with the location in which the light source is installed.
Referring to FIG. 1, a liquid crystal module of the related art using an edge type LED backlight includes a backlight unit 12 to supply light and a liquid crystal display panel 11 to receive the light from the backlight unit 12 to display a picture.
The liquid crystal display panel 11 includes an upper substrate 13 on which a black matrix, a color filter array, a common electrode and an alignment film are sequentially formed; a lower substrate 14 on which a TFT array, a pixel electrode and an alignment film are sequentially formed; an upper polarizer 15 attached to an upper surface of the upper substrate 13; a lower polarizer 16 attached to a lower surface of the lower substrate 14; a spacer (not shown) for maintaining a gap between the upper and lower substrates; and a liquid crystal injected into a space between the upper and lower substrates 13 and 14. The upper substrate 13 and the lower substrate 14 are bonded together by a sealant.
The backlight unit 12 of the related art includes a light emitting diode (hereinafter, referred to as ‘LED’) 20 to generate light; a printed circuit board (hereinafter, referred to as ‘PCB’) 21 on which the LED 20 is mounted; a housing 22 encompassing the LED 20 and the PCB 21; a light guide panel 17 for redirecting light emitted from by LED 20 to illuminate a surface of the liquid crystal display panel; a reflection plate 18 located at the lower part of the light guide panel 17 for reflecting the light traveling to the lower and side surfaces of the light guide panel 17 to the upper surface of the light guide panel; and optical sheets 19 for controlling the diffusion and path of light passing through the light guide panel.
The LED 20 is a self-luminous device. The light generated by the LED 20 is determined by the amount of current supplied to the LED 20. The light generated by the LED 20 illuminates the light guide panel 17 at an incidence surface of the light guide panel 17.
The housing 22 has a reflective inner surface to reflect the light from the LED 20 to the incidence surface of the light guide panel 17.
The reflection plate 18 is installed to face the rear surface of the light guide panel 17. The light guide panel 17 distributes the light incident from the LED 20 to locations at a distance from the LED 20.
The reflection plate 18 re-reflects the light incident on the reflection plate 18 from the rear surface of the light guide panel 17, thereby acting to reduce light loss. The light from the LED 20 is reflected at a designated tilt angle by the reflection plate 18 to the rear surface of the light guide panel 17 to travel to an exiting surface of light guide panel. The light traveling to the lower and side surfaces of the light guide panel 17 is reflected at the reflection plate 18 to travel to the exiting surface of the light guide panel 17.
Optical sheets 19 control the diffusion and direction of travel of light exiting through the exiting surface of the light guide panel 17. To this end, the optical sheets 19 include a diffusion sheet for diffusing the light exiting through the exiting surface of the light guide panel 17 to an entire surface of the liquid crystal display panel; a prism sheet for controlling the direction of the light exiting from the light guide panel 17; and a protection sheet for protecting the prism sheet.
The liquid crystal module includes structures such as a support main for supporting the liquid crystal display panel 11 and the backlight unit 12 and protecting the components from external impacts; a case top; and a cover bottom.
A problem associated with the LED backlight unit 12 of the related art is described below with reference to FIG. 2.
FIG. 2 is a plan view showing an arrangement structure of LEDs 20 mounted on a PCB 21.
Referring to FIG. 2, a plurality of LEDs 20 is arranged on the PCB 21. The LEDs 20 are arranged in a series-parallel connection structure. In such an arrangement structure, a difference in characteristics of the LEDs results in a difference between the amount of a current (I1) flowing through the LEDs 20 in a group A and the amount of a current (I2) flowing through the LEDs 20 in a group B. The difference in the current amounts flowing in the groups of LEDs results in a difference in the light emitted by the groups of LED 20. The difference in emitted light between group A LEDs and group B LEDs results in non-uniform brightness in the light incident on the liquid crystal display panel 1, as shown in FIG. 3.
The use of current mirror circuits has been proposed as a solution to problems associated with differences in LED currents. However, current mirror circuits use a plurality of semiconductor devices. The semiconductor devices increase the cost of liquid crystal display devices.